The object of the proposed study is to determine the biological attributes and function of gut associated lymphoid tissues. Through the proposed study, information will be gained toward the understanding of immunity to enteric disease and the means of enhancing it. Also, the development of allergies associated with the alimentary tract, and the possible mandatory presence of the autochthonous flora for expanding lymphoid cell populations will be elucidated. A promising animal model for studying cellular immune responses to infections in the gastrointestinal tract has been developed using Listeria monocytogenes given orally to gnotobiotic and conventional mice and rats. This model will be exploited to elucidate the interactions of macrophages and T-lymphocytes in the Peyer's patches. It is anticipated that such a model will help in defining the life history of the T-lymphocyte in the Peyer's patches and also in the intestinal villi (the intraepithelial lymphocyte). Newly available monoclonal antibodies, specific for T cells, the helper T cell and suppressor/cytotoxic T cell subsets, and NK cells, will be utilized to dissect the host antimicrobial response and elucidate the ontogeny of gut T cells. The effects which the intestinal flora have on the gut lymphoid tissues will be elucidated by studying t cell-dependent responses, such as delayed hypersensitivity and cell-mediated immune responses, in gnotobiotic mice and rats. It is anticipated that such studies will have bearing on human intestinal disorders suspected of being immune response disorders, such as Crohn's disease and ulcerative colitis.